The present invention relates to a capillary suction type heat fumigation apparatus for drawing up a liquid chemical agent to a wick, a part of which is immersed in the liquid chemical agent, and for causing the liquid chemical agent to transpire by heating the upper portion of the wick for the purpose of insect killing, sterilization, deodorization, fragrance diffusion, etc.
Methods of killing insects by heat fumigation involving a system in which a part of a porous wick is immersed in a liquid insecticide whereby the liquid insecticide is drawn up to the wick and is caused to transpire by heating the upper portion of the wick have conventionally been known. For example, a direct heating system is disclosed in Japanese Utility Model Publication No. 25081/1968. However, since decomposition of such a insecticide is rapid in the case of direct heating, there is a general tendency to adopt an indirect heating system. As for such indirect heating systems, a method of heating a wick with a piece of felt or the like interposed between the wick and a heater is disclosed in Japanese Utility Model Publication Nos. 12459/1961 and 22585/1971, and a method of heating a wick by spacing the wick from a heater by a predetermined interval is disclosed in Japanese Utility Model Publication Nos. 26274/1968, 8361/1969, 14913/1970, 19801/1970 and 29244/1970 and Japanese Patent Publication No. 23163/1986.
Japanese Patent Publication No. 23163/1986 proposes use of a liquid insecticide which is produced by dissolving allethrin or an isomer thereof in a hydrocarbon solvent having a high boiling point in a specified range, and use of a porous wick which consists of inorganic fibers selected from porous ceramics, glass fibers, and asbestos which are bound with gypsum and/or bentonite, or a porous wick which consists of an inorganic powder body selected from kaolin, talc, diatomaceous earth, pearlite, bentonite, alumina, silica, silica alumina and titanium which is bound with starch.
In the case of the above-described heat transpiration method having a wick heating system, since the porous wick is generally made of a felt, unwoven fabric, asbestos or the like, the speed at which liquid is drawn up is comparatively high, so there is a tendency that, as the wick is heated, only the solvent in the liquid chemical agent is evaporated, thereby making it difficult to sufficiently evaporate the chemical agent, and since the high boiling point substances produced by the thermal decomposition of the chemical agent and the high boiling point substances contained in the solvent are apt to cause clogging of the wick, it is difficult to maintain stable evaporation of the chemical agent for a long time.
The use of a wick having a specified composition, in particular, a wick formed of an inorganic powder body such as that shown in Japanese Patent Publication No. 23163/1986, alleviates to some extent the problem whereby only the solvent in the liquid insecticide evaporates and stable evaporation of the insecticide is difficult, but it is still unsatisfactory. In the case of allethrin, it is required that a comparatively large amount of stock is evaporated per unit time in order to obtain a sufficient insect killing effect. Therefore, Japanese Patent Publication No. 23163/1986 proposes a method of heating the upper side surface portion of a porous wick in a comparatively high temperature range of 130.degree. to 140.degree. C. However, heating a wick in such a comparatively high temperature range disadvantageously accelerates the thermal decomposition or polymerization of the chemical agent, thereby reducing the amount of effective evaporation ingredient. In addition, high boiling point substances produced by the thermal decomposition or polymerization are apt to be stored in the wick, thereby causing the wick to clog.
It is known that generally in fats and oils, oxygen reacts with carbon at .beta. position to form hydroperoxide and generate ketone, carboxylic acid, or alcohols. When fats and oils are oxidized in this manner, they become viscous so that the heat conduction is lowered and oil foots and agglutinative substances are apt to be produced. Similarly, when a chemical agent is put into a solvent and is heated, decomposition or polymerization is caused, thereby producing agglutinative substances.
In a draw-up type heat transpiration method, since a liquid chemical agent is drawn up to a wick and the upper portion of the wick is heated to a high temperature, a phenomenon the same as or similar to the above-described phenomenon is caused, so that clogging of the wick and bad heat conduction tend to result in defective transpiration of the chemical agent. In order to improve such defective transpiration of a chemical agent caused by the decomposition or polymerization of the chemical agent due to heating, addition of an antioxidant to the chemical agent is generally suggested. For example, Japanese Patent Publication No. 12106/1979 discloses addition of BHT to a liquid chemical agent.
However, there is a close correlation between the useful life of an antioxidant and the temperature of use, and with a higher temperature, oxidation accelerates rapidly, so that a large amount of antioxidant is used, thereby greatly shortening the useful life of the antioxidant. In addition, since the antioxidant itself is thermally decomposed or evaporated depending upon the temperature condition during use, it may be lost by evaporation or may not be able to manifest antioxidant action. In order to obtain an effective antioxidant action, it is necessary to add more than a predetermined amount of antioxidant, and a large amount of antioxidant is therefore required so that the antioxidant is contained in the total liquid chemical agent.
Some antioxidants only sparingly dissolve in the solvent used, and other antioxidants do not dissolve at room temperature, thereby requiring heating for dissolution, so it is impossible to add a necessary amount of antioxidant by dissolving it.
In addition, thermal deterioration such as color change, or thermal decomposition of a wick composition (in particular, an organic powder) may occur due to friction, drying and heating during the formation of the wick or at the time of using it under heating, which tends to have a deleterious influence on the time stability and the strength of the liquid chemical agent.